Thanks for reading. My question is about a specific step in the proof of the theorem used to calculate the distance between a point and a plane. The proof proceeds up to this point:
And from here the textbook reads as follows:
Since the point $Q(x_1,y_1,z_1)$ lies in the given plane, its coordinates satisfy the equation of that plane; thus
$$ax_1 + by_1 + cz_1 + d = 0$$
$$d = -ax_1 - by_1 - cz_1$$
Substitute this expression in the above equation
Why is this substitution a valid step? I don't understand what "$d$" signifies. And do I simply replace everything above the divide?